1. Field of the Invention
This invention relates to tier sheets and a method for producing the same. More particularly, the invention relates to tier sheets which are used for the conveying operation of various kinds of receptacles such as cans and bottles.
The tier sheet according to the present invention is characterized in that it comprises a first zone having a first frictional resistance and a second zone having a second frictional resistance, wherein the second frictional resistance is lower than the fist frictional resistance. It is preferable that the first zone having first frictional resistance which is higher than the value of second frictional resistance, is formed at least in peripheral regions of the tier sheet.
2. Description of Prior Art
The tier sheet of this kind is suitably used for the palletizing and depalletizing of receptacles and containers. The tier sheet can prevent receptacles from slipping down during conveying or transporting operation, thereby improving the safety and efficiency of such a kind of work.
The palletizing operation generally comprises the following steps:
(1) A certain number of rows of empty cans are placed on a conveyor, and they are pushed forth on to a tier sheet. Another tier sheet is then put on the cans and another layer of cans is again put on the tier sheet, thereby stacking the layers of the cans to prepare a unit load. PA1 (2) The layered unit load is applied with bands such as those made of polypropylene. PA1 (3) The banded unit load is then wrapped with a shrinkable film. PA1 (4) The unit load which is wrapped with a shrinkable film is then placed in a storage or conveyed by a forklift truck. PA1 (5) The unit loads are then transported by a motortruck. PA1 (1) A transported unit load that is wrapped with a shrinkable film is opened. PA1 (2) The bands applied on the load are taken off, and PA1 (3) Each layer of goods in the unit is shifted one by one onto a conveyor and sent to a next operation site such as a filling factory.
The depalletizing operation comprises the steps which are reverse to the above steps, that is:
In recent years, various kinds of receptacles are used in several fields of industry. Receptacles made of metals such as aluminum and steel are used for refreshing drinks, beer, edible oil and canned goods. Glass bottles are used for pharmaceuticals, industrial chemicals and liquid seasonings. Plastic receptacles are used for liquid detergents, food additives and ice cream. Most operation for handling these goods are mechanized or automated.
As a mode of handling and conveying these receptacles, there is proposed a unit load system. In this system, when receptacles are conveyed, a certain number or weight of the receptacles are arranged into one unit and the unit is conveyed intact by transporting machines without unpacking the unit in the route of conveying. That is, a certain number of receptacles are arranged in a plurality of rows and they are stacked in layers, which is followed by wrapping with a shrinkable film and banding for conveying operation.
When empty receptacles are arranged in layers and formed into a unit of load for the purpose of washing, sterilizing or conveying operation, fiber boards or plastic sheets are interposed between adjacent layers of receptacles in order to stabilize them, to avoid the breakdown of the load and to prevent the load from contamination with sundry germs. These sheets which are inserted between layers of goods are called as "tier sheets".
In the palletizing or depalletizing operation or in the washing or sterilizing of receptacles, the tier sheet of this kind (hereinafter sometimes referred to as "sheet", simply) must be slippery because receptacles are caused to slide on the tier sheet. On the other hand, it is necessary that the receptacles must not be slipped down from the tier sheet during the conveying operation with a conveyor, forklift truck or motortruck. Accordingly, conflicting properties to allow slipping and to avoid slipping are simultaneously required of the tier sheet.
As the tier sheets, paper boards were generally used in the conventional art. However, the mechanical properties such as rigidity in bending and shock resistance are reduced when they absorb water or moisture. So that, they cannot be used repeatedly, which is disadvantageous in view of durability. In addition, because such a tier sheet is made of paper fiber, it is liable to become fuzzy which undesirably collects germs and dust. Furthermore, water washing and treatment with hot water are impossible, and satisfactory dust removal or sterilization cannot be done by means of washing with air shower or brushing, flaming or hot roll treatment. Therefore, the contamination of receptacles cannot be avoided and the use of paper-made tier sheet had been a problem in the fields of foods and pharmaceuticals.
As a measure to solve the above-mentioned problem, proposed in International Patent Publication WO 82/01861 is a tier sheet made of plastics which consists of a random copolymer of 2-10% of ethylene and 90-98% of propylene.
However, the conventional plastic-made tier sheet is provided with one smooth surface (specular surface), or with one smooth surface and the other rough surface. When receptacles are put on a rough surface, they are liable to slip because the coefficient of friction between receptacles and the rough surface of the sheet is low, which causes the slip down of receptacles during conveying. Method to avoid such slipping are proposed in Japanese Laid-Open Publication No. 50-136189 and No. 50-143686.
Meanwhile, if receptacles are put on smooth surface, especially on an entirely specular surface, the tier sheet is dragged during the loading or unloading in palletizing or depalletizing operation because the coefficient of friction between the receptacles and the tier sheet is high. As a result, receptacles are tumbled with causing troubles in handling.